1. Field of the Invention
This invention relates to a heat pipe heat exchanger and a method for fabricating the same, in more particularly to a heat pipe heat exchanger in which a heat conductive member is disposed around a heat pipe and a method for fabricating the same.
2. Description of the Related Art
As an example of conventional heat pipe heat exchangers, a heat exchanger comprising a heat pipe composed of a tubular closed container having a perfect circle cross section and enclosing a hydraulic fluid, and a heat conductive member (e.g. fins and/or a heat block) attached to the heat pipe for facilitating heat exchange with the heat pipe has been known.
Such a heat pipe heat exchanger is broadly used in various fields as a thermal diffuser, since the heat pipe heat exchanger has advantages in that the heat pipe itself has a relatively simple structure and that a large heat transfer can be achieved in a small temperature difference.
Conventionally, in such a heat pipe heat exchanger, various methods such as press-fit method, mechanical tube expansion method, charging (filling) method, heating tube expansion method are adopted as a method for attaching the heat conductive member around the heat pipe. In particular, the heating tube expansion method is a method superior to the other methods, since a manufacturing workability can be improved and manufacturing facilities can be miniaturized. For example, the Japanese Patent No. 2541056 discloses a heat pipe heat exchanger using the heating tube expansion method.
Next, the heating tube expansion method is explained in more detail. According to the heating tube expansion method, a pipe-holding hole having a diameter slightly larger than that of the heat pipe is formed at a heat conductive member (e.g. a heat block and/or fins), then the heat pipe (closed container) is installed in the pipe-holding hole. Thereafter, by heating the heat pipe, the heat pipe is plastically deformed by a vapor pressure (internal pressure) of a hydraulic fluid enclosed in the container such that the heat pipe and the heat conductive member are contacted closely with each other and joined with each other. Through the description of the present invention, the “pipe-holding hole” means a hole (aperture) to make contact closely and adhere the heat pipe to the heat conductive member (such as the heat block or the fins).
When the heat pipe is installed in the heat conductive member by using the heating tube expansion method, smaller a gap (space) between an outer periphery surface of the heat pipe and an inner periphery surface of the pipe-holding hole is, smaller an amount of plastic deformation of the heat pipe required for the adhesion is, so that a heating temperature can be lowered. However, considering a required processing precision (processing tolerance) and a manufacturing (assembling) workability of an outer diameter of the heat pipe and an inner diameter of the pipe-holding hole, a tolerance in designed dimensions in a radial direction is generally set around 0.5 mm (a difference in diameters).
On the other hand, in the heating tube expansion method, the heating temperature of the heat pipe should be necessarily set at a temperature that does not go beyond a bursting temperature of the heat pipe.
For example, FIG. 1 is a graph showing a relationship between a heating temperature T and a pressure P as well as a relation between the heating temperature T and an increment ΔD of an outer diameter of the heat pipe, wherein a heat pipe made of copper having an outer diameter d of 9.52 mm, a wall thickness t of 0.34 mm, and an effective length of the heat pipe is 1000 mm and aqua is used as hydraulic fluid. As clearly understood from FIG. 1, a bursting temperature of the heat pipe corresponds to an intersection (about 304° C.) of a curve P1 indicating a saturation vapor pressure of the hydraulic fluid (aqua) in the heat pipe (copper tube) to the heating temperature T and a curve P2 indicating bursting pressure (destruct pressure) of the heat pipe to the heating temperature T.
From the above, when the gap between the outer periphery surface of the heat pipe and the inner periphery surface of the pipe-holding hole is set as 0.5 mm in the case as shown in FIG. 1, the heating temperature T of the heat pipe should be around 300° C. (T≈299° C.), which is slightly lower than a critical temperature (burst temperature), and it is necessary to keep this heating temperature T for a predetermined time. At this time, the internal pressure of the heat pipe becomes about 8.5 MPa (about 85 kgf/cm2).
Now, the heat pipe as described above is fabricated as follows. A pipe for forming a heat pipe is a tube having a perfect circle cross section, and previously provided with a groove (trench) or wick (capillary tube configuration which facilitates a flow back of the hydraulic fluid) at the inner periphery surface thereof. A work of reducing diameter of the tube, etc. is conducted for openings at both ends of the pipe. Thereafter, a reduced opening at one end of the pipe is closed by welding to provide a heat pipe container then a predetermined amount of hydraulic fluid is injected into the heat pipe container. Finally, this inlet (a reduced opening at another end of the pipe) is closed (sealed) by caulking or welding. In the heat pipe fabricated as described above, when the internal pressure is increased by heating, a straight part of the heat pipe having a perfect circle cross section has the highest mechanical strength as against the internal pressure. On the other hand, the both ends of the heat pipe have non-circular cross sections (non-spherical curved surface) since they are provided with the caulking or welding point, so that they have the lowest mechanical strength in the total configuration.
Further, in the heating tube expansion method, when heating the heat pipe installed in the pipe-holding hole of the heat conductive member, the plastic deformation occurs in the heat pipe due to the pipe internal pressure (the vapor pressure of the hydraulic fluid). At this time, the straight part of the heat pipe is protected by being adhered to the inner periphery surface of the pipe-holding hole, since the plastic deformation towards this direction does not progress due to the adhesion. On the other hand, since the ends of the heat pipe are exposed in the air and the mechanical strength thereof is low in the total configuration, there is an apprehension that the ends of the heat pipe may be broken (burst) due to the progress of the plastic deformation.
As described above, in the fabrication of the heat pipe heat exchanger by using the heating tube expansion method, the heat pipe is plastically deformed to be adhered to the heat conductive member around a theoretical destruct point. Therefore, it is very important to determine the gap between the outer periphery surface of the heat pipe and the inner periphery surface of the pipe-holding hole, and to set the heating temperature for the heat pipe. Further, considering a fluctuation in a real heating temperature, etc., the actual work required to be conducted under severe conditions (with small tolerance of margin).
Accordingly, a method for fabricating a heat pipe heat exchanger that can relax the working conditions has been desired.
As described above, according to the conventional method for fabricating a heat pipe heat exchanger, the heat pipe having the perfect circle cross section, which is disposed in the pipe-holding hole having the perfect circle shaped opening, is heated, so that the heat pipe is plastically deformed by the expansion force caused by the vapor pressure of hydraulic fluid in the container, so as to attach the heat conductive member to the heat pipe. Accordingly, it is necessary to heat the heat pipe by controlling the heating temperature to be lower than the critical temperature (bursting temperature) while keeping a plastic deformation amount of the heat pipe. Therefore, there is a disadvantage in that a burden for controlling the temperature on the manufacturer becomes heavy.
In addition, when the heat pipe is heated at a temperature around the critical temperature, a thermal load and a pressure load on the heat pipe are increased. As a result, there are disadvantages in that a rate of defective products becomes high and that productivity (yield) is deteriorated.